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Lecture 4

CHEM 101 Lecture 4: Bohr Model and Quantum TheoryPremium

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Yoram Apelblat

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CHEM 101 - Lecture 4 - Bohr Model and Quantum Theory
Note: Read sections 7.4, 8.1.
Practise problem set 2, questions 1 - 4
Bohr Model for Hydrogen Atom:
Hydrogen atom is the simplest atom
Classical mechanics predicts that the electron would collapse into the positive nucleus,
contrary to observations.
Bohr’s model blends classical and quantum theories.
There is one (and only one) electron moving in a circular orbit around the
The electron has only a fixed set of allowed orbits.
The electron can only have a certain finite energies (also known as quantums)
○ En= -Rh (z2/ h2)
R = Rydberg’s constant = 2.178 x 10-18 J
Z = Nuclear charge = number of protons = atomic number
n = integer value, beginning at 1, going to infinity. n cannot equal 0
So for hydrogen, Z = 1
For He+ Z = 2, Li 2+ Z= 3
radius of an orbit = a x n2 where a=0.053 nm (This explains how each
subsequent orbit gets further and further away from the nucleus)
Ex. for n=1: E1 = -Rh x Z2 < 0 (This will be a negative value)
Ex. for n = infinity: E = -Rh x( Z2/
2) = 0
For absorption: Change in energy is a positive value
For emission: Change in energy is a negative value
Change in energy and wavelength is inversely proportional
Atomic Spectra: Hydrogen and Hydrogen like Atoms
H atom: 1 electron, one proton, z = 1
Bohr model: 1 electron orbits around a nucleus
H -like atoms are really ions
Ex. He has 2 electrons and 2 protons, which does not conform to the Bohr
However, He+ has 1 electron, 2 protons, Z = 2. This follows Bohrs model.
Li 2+ has 1 electron, 3 protons, z =3. This follows Bohrs model.
Li and Li+ will not work because there is more than one electron orbiting around
the nucleus.
General formula for H - like atom: E+ (Z -1)
Only certain energy levels are allowed.
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